[hibernate-dev] "Service" in Hibernate Search: history, lessons learned and rewrite

[Gunnar Morling]

Hey Sanne,

great write-up!

2016-04-27 0:04 GMT+02:00 Sanne Grinovero <sanne at hibernate.org>:

> The "Service" and "ServiceManager" concepts in Hibernate Search have a
> specific meaning which is often misunderstood and/or abused, causing
> trouble.
>
> They also changed over time: victim of two major refactorings which
> evolved the purpose and stretching its intent
>
> So I'll change the definition again :-)
> But hopefully clarifying it, so here is a draft of the rules which I
> plan to both implement and document carefully on the javadoc, with
> some comments to highlight what is changing:
>
>
>
> # A service type is identified by a Class: a Service interface
>
> Nothing new here. Yet: worth stressing that this implies that only one
> implementation will be around.
>
> # There can only be ONE IMPLEMENTATION of a service type used by the
> whole Hibernate Search instance
>
> In other words: it was not a good idea to have the
> LuceneWorkSerializer to be modelled as a Service, back when we
> supposedly could use a different serialization strategy for a
> different index.
>
> Yet it is a good idea nowadays to have LuceneWorkSerializer extend
> Service, as we dropped that level of flexibility. This implies that
> there's a single type of serializer (at most) and it's totally fine to
> expose this as:
>
>    SearchIntegrator#getLuceneWorkSerializer()
>
> [this method doesn't exist yet, but I'm thinking of adding it for our
> convenience and the following other points]
>

It's not clear to me when you'd provide such convenience getter and when
not. What's the criteria for it? It appears you consider different
categories of services?

Personally, I'd like if each service would obtain the other services it
depends on through its constructor. Then it can store the dependencies in
(final) fields and reference them as needed without any sort of look-up.
Services would not have to deal with the service manager anymore, they'd
just get those services they need. One thing I dislike about the current
approach is that you cannot make services fully immutable via final fields
if you depend on other services.


>
> P.S. we're only maintaining - and bundling - a single Serializer
> implementation so it's no surprise that we can handle only one.. yet
> this implies people wanting to override it have to either hack our
> bootstrap or physically remove our implementation.
>
> # A Service implementation can be provided by having it injected at
> bootstrap (i.e.
> org.hibernate.search.cfg.spi.SearchConfiguration.getProvidedServices()
> )
>
> Not a new rule either: repeating for clarity. We call these "provided"
> services.
>
> # If a service isn't "provided", then we attempt to create one using
> java.util.ServiceLoader
>
> Currently this expects a single implementation to be available:
> there's no way to pick which one if there are multiple implementations
> on the classpath.
> I think we'll need to be able to pass a "hint" or similar to the
> requestService to allow expression of preferences, handle shortnames,
> etc.. a proposal for that will follow when there will be need: at this
> point it's important to clarify the limitation, as this expresses what
> a Service is not able to model today.
>

+1 for shaping requirements around this once it's needed. Atm. it appears a
bit blurry to me.

>
> Currently implementations are looked up "on demand". I plan to allow
> "pre-initialize" services as it removes some trouble; these components
> could have convenience getters, not least to remove the concurrency
> overhead.
> Remember that since there's only one implementation for a given type
> around, there's no reason to not do this: the intent of the Service
> contract is to allow people to inject a customized implementation.
>

Sounds good.

# If a Service implementation also implements Startable, or Stoppable,
> we'll invoke the respective methods once at start and/or at stop of
> the Search instance - unless they are provided in which case they are
> ignored.
>
> The current javadoc suggests that it's illegal for a provided
> implementation to also implement Startable and/or Stoppable; I don't
> remember why that was, but today it seems unfitting: people might want
> to extend one of our implementations, or reuse some of the
> implementations normally auto-started but reuse them "by instance" by
> providing them to multiple Search instances to save memory (we
> actually have a need for this for Index Affinity in Infinispan).
> The important concept which will survive, is that we don't start or
> stop stuff which is provided as that's clearly responsibility of
> another component.
>

Why is this? Above you say it's unfitting? I'm not quite clear on the
proposed direction.

# All non-provided Services will be stopped once, and only once as
> final step when the SearchIntegrator is stopped.
>
> This is a significant difference with today's code: we expect the
> Service consumers to "open / close", hopefully in a finally block, to
> the point that Gunnar enhanced it to at least allow AutoClose
> semantics.
> Yet, I don't want runtime code to open and close these frequently as
> it has been a bottleneck in the past.
>

+1 for setting up the graph once and avoiding all the request/release
hassle. Ideally with "constructor injection" as described above.

>
> It also led to the creations of issues like HSEARCH-1589 : we might
> start/stop the same service frequently, and need to improve with
> reference counters.
>
> I suspect that historically the reasoning was to make sure that the
> order of teardown would follow the inverse order of bootstrap as
> components would cleanup after themselves, but having clarified that
> Service instances are unique globally, should also imply that their
> state doesn't depend on other Services. So the teardown order doesn't
> matter anymore.. we'll start one for each, but only close it at
> shutdown.
>

I'm not following on that one. Provided one service uses another and stores
it in a field, tear down order may matter indeed (if a service is using
another one in its close() logic). It'd be resolved easily if circles are
forbidden.

# Hierarchy?
>
> We've talked about global components so far. It's clear that the
> IndexManager has a central role in the overall architecture, as we
> tend to allow per-index customisations. Or per-family customisations,
> as suggested in my previous email.
>
> An example which affects Service:
> The "JestClient client" [the Service we use to connect to
> Elasticsearch] could be considered a good fit for being a "Service" as
> this allows people to override the client implementation and/or inject
> a pre-configured instance.. yet it's not a good fit if we want to
> allow people to connect to different hosts for different indexes.
>
> I don't plan to implement the hierarchical ServiceManager right now,
> but proposing it already so that we can agree on the above cleanups in
> contract, with the perspective that there are cleaner solutions also
> for the scoped use case.
>

It's an interesting idea. The granularity of the hierarchy would have to be
discussed. Would you e.g. have one service manager per index which inherits
the global one?

Implementing these changes resolves or obsoletes at least 10 JIRA
> issues in one shot..
>

What is your plan for this? Which ones do you plan to address as of 5.6,
(5.7), 6 or farther in the future?


> Thanks,
> Sanne
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